Many communication systems including cellular, satellite, and point to point communication systems use a wireless link comprised of a modulated radio frequency (RF) signal to transmit data between two locations. The use of a wireless link is desirable for a variety of reasons including increased mobility and reduced infrastructure requirements when compared to wire line communications systems. One drawback of using a wireless link is the limited amount of communication capacity that results from the limited amount of available RF bandwidth. This limited communication capacity is in contrast to wire-based communication systems where additional capacity can be added by installing additional hardwire connections.
Recognizing the limited nature of RF bandwidth, various signal processing techniques have been developed for increasing the efficiency with which wireless communication systems utilize the available RF bandwidth. One widely accepted example of such a bandwidth-efficient signal processing technique is the IS-95 over-the-air interface standard and its derivatives such as IS-95-A (referred to collectively hereafter as the IS-95 modulation schemes) promulgated by the Telecommunication Industry Association (TIA) and used primarily within cellular telecommunication systems. The IS-95 modulation scheme incorporates code division multiple access (CDMA) signal modulation techniques to create multiple communication channels simultaneously over the same RF bandwidth. In a CDMA cellular telecommunication system, multiple signals are transmitted simultaneously at the same frequency. Such a CDMA system is disclosed in U.S. Pat. No. 4,901,307 to Gilhousen et al., entitled “Spread Spectrum Multiple Access Communication System Using Satellite or Terrestrial Repeaters” and in U.S. Pat. No. 5,103,459 also to Gilhousen et al., entitled “System and Method for Generating Signal Waveforms in a CDMA Cellular Telephone System”, both of which are incorporated by reference. In this type of system, a particular receiver determines which signal is intended for it by a unique spreading code in the signal. Signals at that frequency, without the unique spreading code, appear as noise to that receiver and are ignored. Allowing multiple communication links over the same bandwidth increases the total number of voice calls and other communications that can be conducted in a wireless communication system by, among other things, increasing the frequency reuse in comparison to other wireless telecommunication technologies.
CDMA modulation employs a spread spectrum technique for the transmission of information. In a spread spectrum system, the transmitted signal is spread over a wide frequency band in a pseudorandom fashion. This frequency band is typically substantially wider than the minimum bandwidth required to transmit the signal. The spread spectrum technique is accomplished by modulating each base-band signal to be transmitted with a unique wideband spreading code. Using this technique, a signal having a bandwidth of only a few kilohertz can be spread over a bandwidth of more than a megahertz. A form of frequency diversity is obtained by spreading the transmitted signal over a wide frequency range. Since only 200-300 kHz of a signal is typically affected by a frequency selective fade, the remaining spectrum of the transmitted signal is unaffected. A receiver for the spread spectrum signal, therefore, will be affected less by the fade condition.
In prior art cellular telecommunication systems using CDMA, each voice circuit was assigned a unique code and transmitted on a single channel simultaneously with other voice-coded circuits using the spread spectrum technique. The receiver recovered the signal from the noise by using the same code as the transmitter. Using CDMA, even very low power signals could be recovered by the receiver. Later CDMA systems include data transmission capability, such as electronic mail, facsimile, and Internet access, over additional channels, for use by the subscriber.
A highly simplified CDMA cellular telecommunication system is shown in FIG. 2. Three telephone subscriber mobile units 12a-c are shown along with a single base station 10 within cell 100. A cell is generally defined as a particular RF coverage area and as a mobile changes location, it will possibly move from one cell to the next. Base station 10 is a multicircuit transceiver located at the center of cell 100 whose primary purpose is to handle incoming and outgoing calls within the cell. Calls originating from a particular mobile will be relayed through base station 10. All mobile communications occur through the base stations of each cell via RF transmission, as well as through a mobile telephone switching office (MTSO) computer that is wired to each base station. Reverse channel link R represents RF communication from mobile 12a to base station 10, while forward channel link F represents RF communication from base station 10 to mobile 12a, when mobile 12a is online.
The forward CDMA link contains one or more code channels for communication that are transmitted on a CDMA frequency assignment using a particular pilot pseudorandom noise offset. The frequency assignment is a 1.2288 MHz segment of spectrum centered on a specified channel. Current IS-95 systems always include a pilot channel, possibly one sync channel, as many as seven paging channels, and up to 63 traffic channels, providing that the total including the pilot does not exceed 64, in the forward link.
While earlier CDMA cellular telephone communications did not provide multiple channels to the subscriber in the reverse link, the reverse link of third generation CDMA cellular telephones, for example, CDMA 2000 and W-CDMA (wideband CDMA), provide different channels for access, signaling, voice, and data communications. The different channels available in the reverse link provide a more versatile and efficient communication device for the subscriber.
Attention is directed to FIG. 3 for a background discussion of reverse link CDMA channels for spreading rates 1 and 3. Spreading rate (SR) is the pseudorandom noise (PN) chip rate of a direct-sequence carrier. SRI is 1.2288 MHz and is commonly referred to as 1.times. SR3 is three times this rate, or 3.6864 MHz and is commonly referred to as 3.times. FIG. 3 depicts CDMA channels and their respective “modes of operation” for two modulation schemes, the earlier IS-95 and the later CDMA 2000. Access channel 50 and reverse traffic channel operation 52 are earlier IS-95 modulation schemes. Enhanced access channel operation 58, reverse common control channel operation 64, and reverse traffic channel operation 70 are CDMA 2000 modulation schemes. Many current mobile units implement some of both modulation schemes. In order to transition from the earlier IS-95 to the CDMA 2000 modulation scheme, CDMA 2000 mobiles are being deployed in two design phases. The first phase CDMA 2000 mobiles operate an access channel 50 under the IS-95 modulation scheme, and reverse traffic channel operation 70 under the CDMA 2000 modulation scheme. It is intended that the second phase CDMA 2000 mobiles will replace access channel 50 with enhanced access channel operation 58 and will add reverse common control channel operation 64. The first phase CDMA 2000 mobiles are currently in production using the SR-1 1.2288 MHz (1.times.) PN rate and are operable with radio configurations (RC) 3 and 4 in the reverse link. It is intended that second phase CDMA 2000 mobiles will implement a 3.6864 MHz (3.times.) PN rate and will operate with radio configurations 5 and 6 in the reverse link. RC is the manner in which data bits are built from the voice encoder, or vocoder, and output to the modulator. RC is defined by a “rate set” which includes transmission rate, modulation characteristics, and error correction coding schemes, as defined in the IS2000 standard specifications. For example, RC 1 has a 9600 bits per second (bps) data rate while RC 2 operates at 14400 bps. Radio configurations are typically built into an application specific integrated circuit within each mobile and relate to both the forward and reverse links.
Reverse traffic channel operation 52, enhanced access channel operation 58, reverse common control channel operation 64, and reverse traffic channel operation 70 each have more than one “mode of operation” because a variety of channel modes of operation operate within their respective channel designations. Reverse traffic channel operation 52 operates in either of two modes, as a reverse fundamental channel 54, or up to as many as seven reverse supplemental code, or data, channels 56 in earlier IS-95 systems. Enhanced access channel operation 58 always includes a reverse pilot channel 60 along with an enhanced access channel 62. Reverse common control channel operation 64 always includes a pilot channel 66 as well as the reverse common control channel 68. Reverse common control channel 68 accommodates more than one user at a time and is used for the transmission of user and signaling information to the base station when reverse traffic channels are not in use. Reverse traffic channel operation 70, operable for RC 3 to RC 6, always includes pilot channel 72 and a power control subchannel 80. Reverse traffic channel operation 70 can also contain zero or one dedicated control channel 74, zero or one reverse fundamental channel 76, and zero, one, or two reverse supplemental channels 78. Thus, reverse traffic channel operation 70 operates as one of many possible combinations of channels. First and second phase CDMA 2000 channel modes of operation are described further below.
Although the pilot channel is identified separately at 60, 66, and 72 in FIG. 3 for ease of reference, there is only one pilot channel. The pilot channel operates as a pilot for either enhanced access channel operation 58, reverse common control channel operation 64, or reverse traffic channel operation 70, depending on which channel designation is currently being operated. The pilot channel provides timing information from the mobile for a coherent link between the mobile and base station.
Access channel 50 is used for short signaling message exchanges such as call origination, responses to pages, and registrations, and is a slotted random access channel. During access to the base station, the mobile begins with a calculated transmission output power and steps up the power using a “slotted aloha” protocol until the base station is able to demodulate the signal and acknowledges this to the mobile. At that point the output power is set and a service configuration is negotiated. Enhanced access channel operation 58 is presently intended to be available in future releases of CDMA 2000 and will provide improved system capacity when in use.
Mobile units use the reverse common control channel 68 to sign in with the base station, gain access, and register (identify mobile location and parameters to the base station). This channel will also allow short data bursts commonly referred to as a “short messaging system” (SMS). Dedicated control channel 74 is used only for ongoing signaling information such as pilot strength, pilot set, system parameters, updates, and hand-off messages. Voice, or signaling, can be transmitted over the fundamental channels 76 by various multiplexing options. Alternatively, the fundamental channels 76 can be used exclusively for voice transmission by allocating all signaling to the dedicated control channel 74. Up to two supplemental, or data, channels 78 are used solely for data transmission. At present, each supplemental channel 78 can be set to a data rate between 1.2 kbps and 1.037 Mbps depending upon the proximity of the base station, available power, and number of other users within the cell. Power control subchannel 80 is commonly referred to as a subchannel because it is “punctured” onto the pilot channel and controls the base station power to the mobile, in the forward link. In other words, at a particular time the pilot bits are temporarily replaced with a code instructing the base station to increase or decrease power to the mobile as needed.
Each channel mode of operation (referred to hereinafter as “channel”) in the reverse link is provided with coding for spreading the signal transmitted over the channel and for distinguishing each mobile from all others in a cell. First, a short code is overlaid onto the pilot to provide pseudorandom spreading in the reverse link, and then a long code is overlaid to separate the mobile code from all others. This is accomplished through Hybrid Phase Shift Keying (HPSK), also known as Orthogonal-Complex Quadrature Phase Shift Keying (OC-QPSK), and the use of orthogonal Walsh codes. An example of this process is depicted in FIG. 4, a communications mapping diagram set out in the TIA IS-2000 specification, for a particular set of reverse link channel modes of operation: reverse pilot 60 (66 and 72), reverse dedicated control 74, reverse fundamental 76, reverse supplemental 78′, and either of another reverse supplemental channel 78, reverse common control channel 68, or the enhanced access channel 62, operable with RC 3 or 4. In FIG. 4, binary zeroes are mapped with a +1 and binary ones are mapped with a −1, unused channels and gated-off symbols are represented with zero values, and when the reverse common control or enhanced access channels are used, the reverse pilot channel is the only other channel available. “I-channel” refers to in-phase channel and “Q-channel” refers to quadrature phase channel, the information on each being separated by 90.degree.
Each of the different channel modes of operation described above can come from different base data rates, hence the processing gain (for a constant bandwidth) protecting the signal in the over-the-air channel varies, and the signal-to-noise ratio required at the base station in order to decode the signal varies. Processing gain is the ratio of the bandwidth of a spread spectrum signal to the data rate of the information being transmitted. High processing gain is preferred for providing high system capacity and better quality communication links. Generally, access, pilot, fundamental and dedicated control channel modes of operation have a lower data rate, relatively high processing gain, and a lower signal to noise requirement at the base station. Conversely, each data, or supplemental, channel mode of operation generally has a higher data rate, relatively low processing gain, and a higher signal to noise requirement at the base station. One problem that arises in CDMA communication using multiple channels is that the base station does not know the amount of remaining power that the mobile unit can apply to a lower processing gain channel when the mobile is in communication with the base station on a higher processing gain channel. Consequently, when a lower processing gain channel does come into operation, such as for data transmission, the call may be dropped or high error rates may occur due to the additional power required to transmit the signal over the low processing gain channel exceeding the power transmission capacity of the mobile unit.
Prior art patents discuss power control in CDMA systems but do not address the above problem that occurs when the transmission power required increases due to the addition of one, or more, channels. U.S. Pat. No. 5,926,500 to Odenwalder, entitled “Reduced Peak-To-Average Transmit Power High Data Rate CDMA Wireless Communication System” refers to the generation of a CDMA reverse link with channels having varying processing gains and data rates. However, Odenwalder notes that the total transmission rate can be increased either by transmission over a particular channel at higher rates or by multiple transmission over multiple channels, or both, until the signal processing capability of the receive system, the base station, is exceeded and the error rate becomes unacceptable, or the maximum transmit power of the transmit system, or mobile, is reached. Odenwalder does not provide a solution for keeping the system from exceeding the acceptable error rate or maximum transmit power of the mobile which results in dropped calls. U.S. Pat. No. 5,812,938 to Gilhousen et al. entitled “Reverse Link, Closed Loop Power Control In a Code Division Multiple Access System” describes a method for instructing the mobile to increase or decrease its power depending upon the required Eb/No for a particular data rate. (Signal to noise ratio is routinely defined as: Eb/No, where “Eb” represents the energy per digital bit period and “No” represents noise.) Like Odenwalder, this patent also does not address the issue of dropped calls when the addition of a reverse channel causes the mobile to exceed its maximum transmitter power output, but rather only recognizes the need for increasing transmitter power to meet the required Eb/No of the base station receiver.
When the mobile is gaining access to the telecommunication system, it negotiates the channel configuration based solely on its ability to generate the required data rate. The mobile does not communicate its transmission power capability. Power capability is assumed because the mobile is indeed in communication, but it is in communication at this point only because it is using a higher processing gain channel (at or below 14.4 kbps). It will increase the slotted aloha probe only to determine where the higher processing gain channel is successfully demodulated by the base station. Consequently, when a higher data rate, lower processing gain channel comes into operation, there may not be enough power available from the mobile on the reverse link to sustain the communication connection.
This lack of power availability problem also arises when the mobile changes location in relation to the base station or the cell environment changes. For example, after a data rate has been negotiated, the mobile may discontinue transmission of the data channel periodically because there is no data to be transmitted. During this quiescent period, communication between the base station and the mobile is maintained using only the higher processing pilot gain channel. If the mobile changes location during this period, the amount of required transmit power for the data channel may change. If the mobile moves away from the base station, there will be less power available to transmit on the reverse data channel when it is required; if the mobile moves closer to the base station, more power may be available. Other objects, such as large vehicles, in the path of the RF signal, may block signal transmission to and from the mobile, thereby changing the power requirement. When the data channel comes back into operation at the former data rate, the power it demands may cause the signaling channel power to drop below the receiver requirements of the base station and the call may be dropped. Table 1 below provides an extreme example of the relative power required for three channels, pilot, dedicated control, and supplemental, in a signaling-only versus a data transmission communication at 153.6 kbps, for an IS-98D standard, Class III mobile that is transmitting at a constant power output of 23 dBm, operating in band class 0.
TABLE 1Channel Power Required (dBm)Signaling (Control Hold)Active Data (153.6 kbps)Pilot = 17.7Pilot = 11.8Dedicated Control = 21.5Dedicated Control = 11.1 (9600 bps)(9600 bps)Supplemental Data = 22.4
The example in Table 1 demonstrates that during signaling the pilot must transmit at 17.7 dBm to maintain a connection to the base station, and the dedicated control channel must transmit at 21.5 dBm. However, in order for the subscriber to begin transmitting data at the 153.6 kbps rate, the transmission power available for the dedicated control and pilot channels would have to drop substantially, which would result in a lost connection to the base station and a dropped call.
Because the newer CDMA telecommunication systems provide pilot, dedicated control, supplemental control, data, and voice in the reverse link, it is difficult to obtain enough power from currently available RF amplifiers used in mobile transmitters. One way to retain current RF power amplifier designs and remain within the usable linear portion of these amplifiers, is to negotiate the communication data rate in the reverse link based upon the channel or channels being used, so that the addition of a data channel, or channels, to other active channels, does not require transmission power that exceeds available output power. Current telecommunication systems have no method by which the base station is provided with prior knowledge of the mobile transmitter power status during communication over higher processing gain channels. Accordingly, there is a great need for a method that allows the mobile and base station to negotiate a data rate for low processing gain channels while in communication over higher processing gain channels. It would be beneficial if the system could remain within the maximum transmit output power capacity of the mobile when the low processing gain channel comes online and thus avoid dropped calls. This method should additionally allow the mobile to adjust the data rate to remain within available transmitter power when the mobile changes location within a cell, when the mobile moves to a different cell, or when the cell environment changes.